Buffer Question from BR

[silverice]

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when 10 mL of 0.1M HClO is mixed with 10 mL 0.15M KClO, the pH is 7.64. what is the pH after 30 mL of water is added to raise the volume to 50mL.

the options are: 7.51; 7.64; 7.77; 8.26

 

[MedPR]

when 10 mL of 0.1M HClO is mixed with 10 mL 0.15M KClO, the pH is 7.64. what is the pH after 30 mL of water is added to raise the volume to 50mL.

the options are: 7.51; 7.64; 7.77; 8.26

7.51? In your initial mixture you have a more concentrated conjugate base, so the solution will have more OH- than H+, as evidenced in the ph being above 7. The added water will act as an acid and lower the pH a little.

Notice that this is not a buffer because the acid/conjugate base are not in equal amounts.

 

[chiddler]

7.51? In your initial mixture you have a more concentrated conjugate base, so the solution will have more OH- than H+, as evidenced in the ph being above 7. The added water will act as an acid and lower the pH a little.

Notice that this is not a buffer because the acid/conjugate base are not in equal amounts.

Buffer systems do not need to have acid/conjugate in equal amounts!

So given it is a buffer system, as we so demoralizingly found out yesterday with milski's help, pH does not change.

 

[milski]

Buffer systems do not need to have acid/conjugate in equal amounts!

So given it is a buffer system, as we so demoralizingly found out yesterday with milski's help, pH does not change.

Nerd alert: the difference in pH is about 9*10^-6.

 

[bajoneswadup]

I think the PH would normally change had an acid or base been added, except that water doesn't change buffer systems.

 

[MrNeuro]

Nerd alert: the difference in pH is about 9*10^-6.

is it safe to assume that usually any reasonable addition of water to a buffered system is not going to have a significant effect.

in other words any reasonable addition of water to a buffered system will affect pH by < 0.01

 

[milski]

is it safe to assume that usually any reasonable addition of water to a buffered system is not going to have a significant effect.

in other words any reasonable addition of water to a buffered system will affect pH by < 0.01

Yes, if you have both the acid and the conjugate adding reasonable amounts of the solvent will not change the pH. That comes from pKa=[H][A]/[HA]. When you add solvent, you change [A] and [HA] by the same factor.

 

[silverice]

Buffer systems do not need to have acid/conjugate in equal amounts!

So given it is a buffer system, as we so demoralizingly found out yesterday with milski's help, pH does not change.

Did milski said it in a post? I'm having trouble with this buffer thing. It would be helpful if I can read the post.

 

[milski]

Did milski said it in a post? I'm having trouble with this buffer thing. It would be helpful if I can read the post.

http://forums.studentdoctor.net/showthread.php?t=892199

But it's not much different from what I just posted here: the ratio of [A]/[HA] stays constant and from the equation for Ka, [H] has to stay constant as well. The only time that is not exactly true is when the concentrations get so low that the solvent (water) becomes a significant source of [H].

 

[MedPR]

Buffer systems do not need to have acid/conjugate in equal amounts!

So given it is a buffer system, as we so demoralizingly found out yesterday with milski's help, pH does not change.

Huh? I thought a buffer made of weak conjugates needed to have both conjugates in equal amounts?

 

[chiddler]

Huh? I thought a buffer made of weak conjugates needed to have both conjugates in equal amounts?

TBR said that the most it can be is a 1:10 ratio (so log(10) = 1 or log(0.1)= -1 for the purpose of the HH equation). This is also why buffers only work within + or - 1 within their pKa range.

 

[milski]

Huh? I thought a buffer made of weak conjugates needed to have both conjugates in equal amounts?

1:1 will give you the best performance, as in most resistance to changes in pH, but they'll stay quite stable as the ratio increases. It's a gradual process so the 1:10 is somewhat arbitrary but you have to have it somewhere.

 

[MedPR]

TBR said that the most it can be is a 1:10 ratio (so log(10) = 1 or log(0.1)= -1 for the purpose of the HH equation). This is also why buffers only work within + or - 1 within their pKa range.

1:1 will give you the best performance, as in most resistance to changes in pH, but they'll stay quite stable as the ratio increases. It's a gradual process so the 1:10 is somewhat arbitrary but you have to have it somewhere.

Ah that's right. Totally forgot about the pKa+/- 1 rule. Thanks!

 

[Temperature101]

when 10 mL of 0.1M HClO is mixed with 10 mL 0.15M KClO, the pH is 7.64. what is the pH after 30 mL of water is added to raise the volume to 50mL.

the options are: 7.51; 7.64; 7.77; 8.26

Is the answer 7.51? Water donates equal amount of OH- and H+ to both solution. Therefore, water won't affect the buffer at all.

 

[milski]

Is the answer 7.51? Water donates equal amount of OH- and H+ to both solution. Therefore, water won't affect the buffer at all.

The initial pH is 7.64 and does not change, so the answer is 7.64

Water donating the same amount of OH- and H+ is not the correct reason. Water always donates the same amount of OH-/H+. If that was a reason for constant pH, you would not be able to change the pPh of any solution by adding water.

The trick here is that when you're adding water you're changing the concentrations of acid and conjugate by the same factor. That makes their ratio constant and gives you a constant [H]. That will be true even for solving different from water, which contributes only to [H].

 

[gentlebeast]

The initial pH is 7.64 and does not change, so the answer is 7.64

Water donating the same amount of OH- and H+ is not the correct reason. Water always donates the same amount of OH-/H+. If that was a reason for constant pH, you would not be able to change the pPh of any solution by adding water.

The trick here is that when you're adding water you're changing the concentrations of acid and conjugate by the same factor. That makes their ratio constant and gives you a constant [H]. That will be true even for solving different from water, which contributes only to [H].

I would argue that adding water changes ph of weak acid or weak base. Weak acid becomes less acidic and base becomes less basic. Buffer doesn't change bc it contains both.

 

[MedPR]

I would argue that adding water changes ph of weak acid or weak base. Weak acid becomes less acidic and base becomes less basic. Buffer doesn't change bc it contains both.

Well, that's the whole idea behind a buffer. You can stick in some acid, or some base, and the buffer will compensate for it.